Since an oil-free screw compressor does not supply oil into the rotor chamber, the oil-free screw compressor is more likely to leak a compressed gas than an oil supply type screw compressor. This leakage of the compressed gas easily affects the compressor performance. Thus, a shaft seal device (a gas seal) used to prevent the compressed gas from leaking from the area between the rotor shaft and the casing.
The oil-free screw compressor supplies lubricating oil to the bearing that supports the rotor shaft and the gear mechanism that transmits the rotational drive force from the drive source to the rotor shaft. When the rotor chamber has a negative pressure during unload operation, the lubricating oil supplied to the bearing and the like can flow into the rotor chamber. Thus, a shaft seal device (an oil seal) is used to prevent the lubricating oil supplied to the bearing and the like from flowing into the rotor chamber.
An oil-free compressor having the shaft seal device (the gas seal and the oil seal) is disclosed in e.g., Japanese Utility Model Application Laid-Open (JP-U) No. 61-144289.